1. Field of the Invention
The present invention relates to a lubricating structure for engines, a lubricating structure for engines for snow vehicles, and a snow vehicle, wherein dry sump lubrication of a four-cycle engine is performed.
2. Description of the Related Art
In recent years, more and more four-cycle engines have come to be used for snow vehicles, and there have been proposed four-cycle engines including a dry sump lubricating structure and a clutch mechanism implemented e.g. by a constantly variable transmission. A lubricating structure in an engine of this type is generally provided with an oil feed pump for feeding lubricating oil and an oil recovery pump for recovering lubricating oil.
In designing the arrangement of the two oil pumps in the lubricating structure, it is necessary to take into account steering stability of the vehicle, reduction of the size of the engine, prevention of interference with other components, and-so forth. For example, in the case where the clutch mechanism is disposed on the output shaft side of the crankshaft, it is not preferable, with a view to avoidance of interference with the clutch mechanism, that the oil feed pump and the oil recovery pump are disposed on the output shaft side of the crankshaft.
In Japanese Laid-Open Patent Publication (Kokai) No. 2001-280111, it is disclosed that oil pumps are disposed such that the arrangement of an oil filter and communication passages between the oil pumps and the oil filter is designed so as to facilitate machining of oil passages and reduce the size of the engine.
Some snow vehicles with four-cycle engines are provided with a balancer shaft. In such a snow vehicle, the balancer shaft is normally rotatably supported in a crankcase. A balancer shaft requires lubrication, and particularly when two balancer shafts are provided, the arrangement of oil passages for lubrication influences not only the degree of difficulty in passage machining but also space saving within the engine. Therefore, efficient designing of passages is desired.
However, in the above-mentioned dry sump lubricating structure, if the oil pumps are disposed at locations away from the crankshaft, e.g. in a lower part of the engine, and the crankshaft is used to drive the oil pumps, oil pump driven gears of an increased size have to be employed for speed reduction, which hinders saving of space within the engine. Further, in the case where a magnet device is provided on an end of the crankshaft opposite from the clutch mechanism, if it is designed such that the oil pumps are driven by the end of the crankshaft via gears and chains, the crankshaft inevitably has to be lengthened for allowing a driving force to be taken therefrom, which leads to an increase in the overall width of the engine.
Further, in a snow vehicle with a four-cycle engine, if oil passages are formed, for example, such that they extend from a main oil gallery to the two balancer shafts, the oil passages are complicated in structure, which not only makes passage machining difficult, but also requires an engine with an increased size for securing space for machining the passages.
In Japanese Laid-Open Patent Publication (Kokai) No. H08-177484, it is disclosed that a main oil gallery and a sub oil gallery are connected by a branch pipe. However, this needs provision of the additional branch pipe, and therefore it is not suitable for space saving.
Further, an oil filter provided in a snow vehicle with a four-cycle engine requires periodical replacement, and it is, therefore, necessary to dispose the oil filter at a location free from interference with other components and facilitating the replacement operation. For example, if the oil filter is disposed in a rear part (intake side) of a rearward-tilted engine, or if the filter is disposed in a front part (exhaust side) of a forward-tilted engine, the oil filter comes to be located below the engine, and therefore the replacement operation becomes difficult to carry out. Further, if the oil filter is disposed on the magnet device side, the space between the oil filter and a side wall of the engine room is small, which makes it difficult to mount or remove the oil filter. In addition, it becomes necessary to avoid interference of the oil filter with the clutch mechanism, an exhaust pipe, a steering post, an engine mount, and other components.